If the value remains unissued, then it could be considered to be held in reserve for a future multisig retroactive grants program, for example. If there were the desire to do so, that value could also be tracked separately as a “reserve” value separate from the ZSF balance in the protocol. Really, though, there would really be no need to do so; the value of the unissued 20% could be computed exactly if a future network upgrade made multisig-based distribution of that value a possibility. It’s essentially what @skyl describes in Moving the Dev Fund discussion forward - #107 by skyl
The main idea is that there would be no real reason (and that there are in fact nontrivial downsides) to hold such a reserve in UTXOs.